The present invention relates to the completion of nondestructive soft-field tomography analysis of an object under study. In particular, the object of the present invention is a data acquisition and processing method, as well as a measuring system for the soft-field tomographic mapping of the internal structure of objects under study, specifically objects with inhomogeneous material distribution. The present invention, particularly, provides a data acquisition/processing method, as well as a measuring system serving as a basis for a soft-field tomography analysis and/or an imaging method. The method and measuring system according to the invention may be used advantageously in industrial tomography processes, for nondestructive structural analysis of different materials (e.g. live wood), and in medical diagnostic methods (such as e.g. body-fat analysis, in particular, obesity measurement, or oncological examinations performed, in particular, for mapping cell proliferation, and other similar methods). Accordingly, here and from now on the term “object” may substantially refer to any material under study ranging from industrial articles through structures found in nature to organs/tissues forming parts of a human or animal body.
The problem of analyzing or visualizing the internal structure of a three-dimensional object without destroying it is present in numerous fields of technology and science. Said analysis or visualization can be achieved by means of transilluminating the object by a radiation that is mostly transmitted by the object under study, but also absorbed by it to a sufficient extent to obtain a ‘silhouette’ of its internal structure (imaging). If the internal structure of the object under study is nearly two dimensional or is not too complex, such silhouettes can be directly interpreted. In more complex cases, however, images captured in only a few directions are not sufficient for revealing the internal structure. Reconstructing the internal structure of an object (e.g. the material distribution, inhomogeneity, internal defects of said object, etc.) from two dimensional silhouettes is generally a rather complicated task, therefore, it is a common practice to take so-called tomographic sectional images. This means that a planar section of the three dimensional object is studied by capturing one dimensional images in different directions along the plane and then attempting the reconstruction of the planar structure from said images. In this context, the term “structure” refers to the spatial variation/distribution of the absorption of the radiation applied—thus, tomography refers to the solution of this two-dimensional problem. For such, so-called hard-field tomography methods, X-ray radiation or acoustic radiation (e.g. ultrasound) is commonly used as the transilluminating radiation.
Besides the above discussed methods, so-called soft-field tomography methods are also known. Such a soft-field tomography method is, without completeness, amongst others the electrical resistance tomography (ERT), electrical capacitance tomography (ECT), electrical impedance tomography (EIT) and acoustic impedance tomography (AIT), which inspection methods are directed to the mapping and measuring of the spatial variance of the internal properties of the object under study, such as the electrical/dielectric or mechanical properties, e.g. the electrical conductance or electric permittivity or density, of the material defining the internal structure of the object, specifically, by means of performing e.g. magnetic or electrical impedance/admittance or acoustic impedance measurements. To this end, at first the object under study is subjected to excitation through one or more points thereof, and then the response of the material of the object to the excitation is measured at one or more points of the object. The excitation and/or the response measurement can be performed at one or more frequencies and/or instants of time. The image of the object studied is created subsequently by an image reconstruction method on the basis of the thus obtained measurement data.
When further use (for e.g. inspection or diagnostics) of the image created by means of imaging is concerned, it is of critical importance that various errors getting incorporated into the measurement data during data acquisition be compensated to the greatest possible extent. The source of the measurement errors may be e.g. the measuring system used for the excitation and/or data acquisition, the individual excitation- and/or measuring units used in the measuring system, i.e. their systematic errors, or the errors arising from the geometric positions occupied by the excitation and/or measuring units during the measurement. Amongst these errors of soft-field tomographic systems, said systematic error is considered particularly important, because it cannot be measured by a preliminary calibration of the measuring system used for the analysis as due to the nonlinearity present in the system of the object and the measuring system, the object under study also influences the measuring system, and thus one or more parts thereof.
U.S. Publication Pamphlet No. 2010/0127705 A1 discloses a method and system for carrying out tomographic analysis based on magnetic induction. In order to perform the analysis, the object under study is subjected to an alternating magnetic field generated by excitation coils arranged around the object, then alternating current signals carrying information characteristic to the electrical conductance of the object and the spatial distribution of said conductance is captured by suitable receiving units arranged in positions surrounding the object. Data acquisition is carried out at least at two different frequencies. Using the thus recorded measurement data, particularly the imaginary parts thereof, a correction factor is derived through complicated mathematical algorithms, which may then be used for compensating the errors (scattered signals) arising due to a change in geometric positions of the excitation coils and/or receiving units. A drawback of the method is that only the imaginary part of the recorded data is used to derive the correction factor, which causes data loss. Furthermore, the method is also unable to manage systematic errors of the apparatus, the parts thereof, in particular, the excitation coils and/or receiving units.
U.S. Publication Pamphlet No. 2013/0013239 A1 discusses the use of phase modulated or phase- and amplitude modulated excitation generally in soft-field tomographic methods to enhance the resolution of data acquisition/imaging.
U.S. Pat. No. 8,593,154 B2 discloses a method and apparatus to suppress artifacts arising in soft-field tomography investigation. The used data acquisition system 20 is shown in FIG. 1. Accordingly, the electrical properties of the material of the object 22 under study, in particular, its electrical conductivity and the spatial distribution of the electrical conductivity are analyzed by EIT method. To this end, a plurality of transducers are arranged on or near a surface of the object 22. The transducers are used firstly to excite the material of the object 22 in the desired manner (here, electrically) and secondly to capture the response of the material of the object to this excitation and convert it to electrical signals. Accordingly, the transducers are electrically connected firstly to the excitation units 26 providing their drive and secondly to the response detectors 28 for capturing signals representing the response. The signals of the response detectors 28 are passed on to a soft-field tomography module 30 connected to the response detectors 28, whose task is to calculate/evaluate the response of the object 22 to the excitation, and optionally solving the so-called inverse problem connecting the response, the excitation and the electrical conductivity-distribution in the object. The correct operation of the data acquisition system 20 is provided by the control unit 33, which is connected to the excitation units 26 and to the module 30. According to the disclosed solution, the object is subjected to excitation by the transducers 24 according to an excitation pattern comprising a plurality of frequency- and/or time component, then one or more artifacts of the imaging, particularly errors resulting from the geometric arrangement of the transducers and the inability of the electrodes used therein to be fitted to the forward predictive model used in the module 30 are desired to be eliminated by separating the response according to components. The excitation and data acquisition schemes described in detail however do not alleviate the systematic errors caused by the data acquisition system 20 and the subassemblies thereof, moreover these are not even mentioned.
In the soft-field tomography methods, thus particularly also in EIT and AIT measurements, the continuous (inhomogeneous, non-isotropic) medium (optionally comprising structural defects/deformations) is simplified to an impedance (or complex conductivity) network so that it may be managed with the toolset of numerical mathematics. To this end                the material of the object under study is modeled with a linear network, whose mathematical description is solved and more or less well manageable;        the inverse problem is limited to calculating the impedance values on the branches of the network (generating the equation system is unequivocal, thus the inverse problem can be solved with greater safety);        the frequency-dependency of the material of the object under study may be estimated more accurately, because the impedances on each branch of the network may be modeled separately as a connection of electrical resistors, capacitors and inductors.        
In the publication of H. Gagnon et al. entitled “A Resistive Mesh Phantom for Assessing the Performance of EIT Systems” published in 2009. in the magazine entitled IEEE Transactions on Biomedical Engineering in volume 57, Issue 9, pages 2257-2266, consideration regarding the design of a so-called phantom that can be used for the evaluation, comparison and calibration of EIT systems are discussed. The matrix method and the finite element method commonly used in solving electrical circuits is also discussed, and a method is also described for establishing a sufficiently accurate correspondence between the measurable parameters of the object under study and the impedance network modeling the object described by a directed network graph.